1. Field of the Invention
The invention relates to a system for the evaluation of a subcode of a digital information transmission, where the subcode is associated with a large redundancy and comprises several segments, which system is adapted in particular for the evaluation of a subcode in the case of compact disc recordings.
2. Brief Description of the Background of the Invention Including Prior Art
In the case of storage of signals such as, for example, audio signals in a compact disc technique, the so-called subcode channels are recorded at the same time the recording itself takes place. Such subcodes serve for the storage of data which are associated with the main storage data in reasonable connection, for example if the recorded data represent a music program. The subcode can be information which is not intended to be part of the direct proper audio transmission. For example, the so-called Q-channel of a subcode serves for operating and displaying the status and the operation of a compact disc apparatus.
The subcode channels are subdivided into individual so-called blocks, and these blocks are subdivided into segments. In general, a block is a set of locations or tape positions in which a block of words is stored. A segment is in general a portion of a line or of a curve disposed between two points. For example, these points can be points on a compact disc or abstract points. For example, the subcode channel O can comprise 96 bits plus two synchronization bits and comprise information relating to the title, the number, the running time of the piece of music in minutes and seconds, or it can provide information about the contents of an audio record disc. The information bits of a subcode segment can be embedded in the data bits of the audio program in a convoluted way. In order to maintain a sufficiently large redundancy, 75 subcode blocks can be scanned in each second. In this way, a continuous flow of information about the subcode data is possible during the reproduction of a music program. In order to maintain a testing of the code, 16 parity bits can be joined to the 80 data bits of one block in order to be able to detect errors on the reproduction side of the information. Errors can be recognized with the aid of cyclic redundancy checks for example. The parity bits are so joined to the data bits that with the aid of the CRC (cyclic redundancy check), that is, after a division of the contents of the block by a predetermined generator polynominal G(x), there has to result a quotient without a remainder. If the division results in a remainder, then a bit error is present in the data word. It can be easily recognized that in the case of a plurality of bits and in the present case of 96 bits, there frequently arise bit errors such that the read-out information is declared by the evaluation logic as without value and is rejected. A localization of the bit error is not possible. One can easily imagine that a sequence of bit errors is present, for example, in the way of "drop outs" on digital sound recordings. In addition, defective settings at the reproduction apparatus can cause bit errors. Furthermore, bit errors can occur again and again in the subcode region of the Q-channel. This results in a noisy display of the selected information, such as for example, the program title and program number, the running time and so on. However, in the case of the reproduction of a music program, not all information is continuously required. In the evaluation of the accumulated time for example, the other segments of the data block are of no interest. Since, however, bit errors are present in these segments, the cyclic redundancy check testing method recognizes an error of the complete block such that no information is available because the cyclic redundancy check method does not permit a subdivision of the testing within a block. Therefore, with this method, an interference free evaluation is not always possible. In addition, the cyclic redundancy check testing method entails substantial calculating requirements since the operation of dividing by the generator polynominal has to be performed continously.